This invention relates to an improved method and apparatus for producing chemical and physical changes in substances by exposing them to the direct current of an electric arc. More particularly, the present invention relates to a method and apparatus for enhancing the stability of a free-burning electric arc into which various substances are forcefully injected to produce chemical and physical changes in those substances.
Various techniques are known for generating a continuous stream of plasma by means of an electric arc. As would be expected, there are practical problems associated with each technique for generating the plasma. Moreover, each method of generation affects the character of the plasma produced. Utilization of the plasma to bring about physical and chemical changes in various substances is even more complex.
A novel and very effective technique for efficiently energizing reactant materials in the conduction column of a free-burning electric arc involves forcefully injecting a substance along a cathode having a conical tip, into the contraction zone of the conduction column. This technique takes advantage of the so-called cathode jet effect due to the contraction of the arc column near the cathode. In effect, under the right conditions the plasma moves away from the cathode with a concomitant decrease in the pressure at the base of the contraction zone so that the arc in this region aspirates gas from the surrounding atmosphere to form a continuous cathode jet. The contraction zone serves as an "injection window" across which materials may be injected directly into the arc column at substantially improved flow rates without disturbing the stability of the arc by virtue of the forced convection to which it is subjected. This is particularly true when the anode and cathode are directly opposed in a co-linear configuration.
Unfortunately, opposing such a cathode and anode in a co-linear configuration presents myriad of operational problems which must be overcome in order to achieve a commercially reliable process involving the use of an electric arc to promote chemical and physical changes. For example, when the cathode jet is allowed to impinge directly on a non-consumable solid anode the anode displays excessive wear and high heat losses are observed rendering the process generally inefficient. More importantly, if the material fed into the arc contains an entrained solid, the anode surface adjacent to and surrounding the anode spot, i.e., the area of the anode receiving the arc current, quickly becomes coated with a layer of solid. Deposition of the solid on the anode soon interferes with stable arc operation and contributes significantly to anode corrosion.
One technique employed in an attempt to overcome anode corrosion involves raising the current density at the anode spot sufficiently to vaporize the anode material thus generating a copious flow of vapor away from the anode spot. The anodes are consumed in this high intensity arc type of discharge.
In another high intensity arc type of discharge a porous anode is used through which a gas is transpirated to provide a continuous stream of gas away from the anode spot. These porous anodes used in this technique are referred to as fluid transpiration anodes.
Nothwithstanding the ability of the aformentioned technique to keep the anode spot clear of deposited material, extreme difficulty has been encountered with the high intensity mode of operation in controlling the arc effluent. When the anode and cathode are opposed in co-linear configuration, the arc effluent flares radially outward in all directions at the point where the two jets collide and hence a well defined unidirectional effluent jet is not obtained. When the cathode is inclined at a sufficiently large angle to the anode to allow the two jets to merge smoothly the arc column is enormously elongated and the effluent is not well defined. Moreover, when it is desired to quench the arc effluent, the jet of quench gas tends to interfere with the arc stability owing to the elongated column. Consequently, there is a need for improved methods and apparatus for energizing fluid materials in a plasma jet that efficiently utilize the energy of the arc while at the same time maximize arc stability.